1. Field of the Invention
The present invention relates to analog circuits, and more particularly, to stages for amplifier circuits.
2. Description of the Related Art
Amplifiers are commonly known and used in discrete analog circuits and in monolithic integrated circuits (ICs). For details on amplifier fundamentals, see e.g., Malvino, Electronic Principles, McGraw-Hill, Inc., 1973. Often amplifiers will consist of a number of stages. These stages may, for example, include an input stage followed by one or more amplifying stages. The last stage is referred to as an output stage. One function served by an input stage is to provide an appropriate input impedance. The input stage may also provide some amplification. Usually, the amplifying stages (including the output stage) provide most of the needed amplification.
FIG. 1 is a schematic diagram of a conventional amplifier 100. The conventional amplifier 100 has a differential input stage 102 followed by a folded cascode stage 104. The differential input stage 102 is a differential amplifier constructed from a current source I.sub.1 and transistors Q.sub.1 and Q.sub.2. The folded cascode stage 104 is constructed from cascode transistors Q.sub.3 and Q.sub.4, load transistors Q.sub.5 and Q.sub.6, and emitter resisters R.sub.E1 and R.sub.E2. The conventional amplifier 100 also includes an output voltage V.sub.OUT, an output capacitor C.sub.C, and a bias voltage source V.sub.B. The conventional amplifier 100 offers a high bandwidth and essentially a rail to rail voltage swing, both of which are advantageous. However, one problem with the conventional amplifier 100 is that the maximum rate-of-change, i.e., slew rate, for the output voltage V.sub.OUT is significantly limited. In effect, the current source I.sup.t operates to determine a maximum output current i.sub.o that leads to the slew rate limitation. This slew rate limitation is commonly present with amplifiers that employ differential stages such as that illustrated in FIG. 1.
One approach to overcome the slew rate limitation is to use a complementary design. FIG. 2 is a schematic diagram of a conventional amplifier 200. Although other complementary designs exists, the conventional amplifier 200 is at least useful to illustrate the improvement provided by a complementary design. The conventional amplifier 200 is constructed from a diamond follower input stage 202 and a complementary current mirror stage 204. The diamond follower input stage 202 is constructed from current sources I.sub.1 and I.sub.2 and transistors Q.sub.1 through Q.sub.4. The transistors Q.sub.3 and Q.sub.4 drive opposing current mirror circuits formed by transistors Q.sub.7 and Q.sub.8 and transistors Q.sub.5 and Q.sub.6, respectively. The maximum slew rate for the output voltage V.sub.OUT then depends on the amount of output current i.sub.o available to charge a compensation capacitor C.sub.C. Here, however, unlike FIG. 1, the output current i.sub.o is not limited by the current sources I.sub.1 and I.sub.2 but instead offers (via the driving of transistors Q.sub.3 and Q.sub.4) the ability to provide a large and largely unlimited output current i.sub.o. Consequently, the conventional amplifier 200 has a slew rate limit that far exceeds that available with the conventional amplifier 100 illustrated in FIG. 1.
Although the conventional amplifier 200 illustrated in FIG. 2 overcomes the slew rate problem, the conventional amplifier 200 is a non-cascode amplifier and as such the input voltage swing of the conventional amplifier 200 is restricted. The input voltage swing is restricted by two emitter-base voltage drops (i.e., V.sub.be (Q.sub.2) and V.sub.be (Q.sub.5)) for negative going input signals) that separate the input voltage range from the power supply rails (V.sub.CC and V.sub.EE), which in low power designs can be a substantial penalty.
Thus, there is a need for an amplifier design that offers not only high bandwidth operation but also a high slew rate and a wide input voltage swing.